1. Field of the Invention
The present invention relates to a linear actuator intended to be used in a damper and spring unit for a vehicle suspension to allow adjustment of the vertical position of the spring, and hence of the height of the vehicle body from the ground.
2. Description of the Related Art
In a vehicle suspension comprising a damper and spring unit, in which the damper is connected at its bottom end to a wheel-carrier or to a suspension arm and at its top end to the vehicle body and in which the spring is arranged around the damper and rests at its bottom end against a spring plate attached to the damper and at its top end against the vehicle body, it is known to use a linear actuator interposed between the damper and the spring plate to change the vertical position of the spring plate and allow therefore adjustment of the height of the vehicle body from the ground.
WO2009/033985 discloses a damper and spring unit for a vehicle suspension provided with a linear actuator for adjusting the height of the vehicle body from the ground, wherein the linear actuator is interposed between the damper and the spring plate. The linear actuator comprises a cylinder attached to the damper and an annular piston vertically movable relative to the cylinder. The cylinder comprises an inner cylindrical element and an outer cylindrical element which are made as separate pieces. The inner cylindrical element is arranged around the damper, coaxially thereto. A working chamber is provided between the inner cylindrical element and the outer cylindrical element and is supplied with oil under pressure through a fitting. The annular piston is slidably guided in the working chamber. The spring plate is connected to the free end (top end) of the annular piston, whereby a vertical displacement of the annular piston relative to the cylinder results in a corresponding vertical displacement of the spring plate, and hence of the bottom end of the spring, relative to the cylinder, and hence relative to the damper. The linear actuator disclosed in this document is therefore made up of three pieces, namely the inner cylindrical element, the outer cylindrical element and the annular piston. These three pieces define altogether the working chamber, in that the working chamber is enclosed laterally by the inner cylindrical element and by the outer cylindrical element, on its bottom side by the inner cylindrical element and on its top side by the annular piston.
Another example of a linear actuator interposed between the damper and the spring of a damper and spring unit for a vehicle suspension for adjusting the height of the vehicle body from the ground, wherein the linear actuator is made up of three pieces, two of which form the cylinder and the other one forms the piston of the actuator, is known from WO2012/156418 in the Applicant's name. Moreover, according to this known example the spring rests at its bottom end on a spring plate mounted above the piston of the actuator and is therefore wholly arranged above the piston of the actuator.
A linear actuator for adjusting the vertical position of the spring of a damper and spring unit for a vehicle suspension is also disclosed in DE 10 2005 008 814. This known linear actuator is basically made up of only two elements, namely an inner cylindrical element, or cylinder, arranged to be mounted around the cylinder of the damper, and an outer cylindrical element, or piston, which is mounted so as to be axially slidable relative to the inner cylindrical element and is arranged to support the bottom end of the spring. The two cylindrical elements of the actuator enclose, both radially and axially, a working chamber arranged to be filled with a fluid under pressure in order to adjust the axial position of the outer cylindrical element relative to the inner cylindrical element, and hence the vertical position of the bottom end of the spring relative to the cylinder of the damper. According to this known solution, the spring rests on a spring seat provided at the top end of the outer cylindrical element of the actuator, and is therefore wholly arranged above this element of the actuator. Furthermore, this document does neither show nor explain how the movement of the outer cylindrical element in the upward direction is stopped. Moreover, due to the way the spring is supported on the outer cylindrical element, it is very complicated, if not even impossible, to mount a top end-of-travel element arranged to stop the movement of the outer cylindrical element in the upward direction. Therefore, the linear actuator as disclosed in DE 10 2005 008 814 cannot be reduced to practice.